#60secondscience – My Chicago Botanic Garden

60-Second Science: Green Roof Plants are Tough

Gardeners and farmers know that healthy plants need good soil and the right amounts of both water and sunlight. But green roofs are intentionally built with an engineered soil-like substance that more closely resembles a pile of rocks than rich, moist potting soil.

To make matters worse, the tops of buildings are often blindingly sunny and very hot in the summer. So how do plants like grasses and wildflowers survive in this type of harsh environment?

PHOTO: Cactus and allium grown on green roof. — Cactus and other succulents retain water in their tissues. Ornamental onion *(Allium*) species have underground bulbs that help them get through cold winters and dry summers.

Not all plants will grow on a green roof, even in the temperate Midwest. Most plant species that are successful in the desert-like habitats of green roofs have beneficial adaptations that allow them to absorb and store water and nutrients. Some have succulent leaves with thick waxy coatings to prevent water from evaporating. Others have roots that grow horizontally rather than vertically to maximize the areas from which they absorb water and nutrients. Some use a modified type of photosynthesis to prevent water loss during the hottest and driest part of the day. Still others use bulbs or underground tubers to store nutrients during the long cold winters. Some species may even form partnerships with special fungi in the soil that help their roots with more effective absorption.

While plant species evolved to develop these various adaptations on the ground, such traits serve the individual plants very well in the harsh environment of a green roof. The next time you visit a green roof, you might see a striking diversity of species but you won’t see any wimps. No, these plants are both beautiful and tough.

PHOTO: Shortgrass prairie plants grown on a rooftop garden at shallow depths. — Even in very shallow soil and full sun, some plants that normally grow in shortgrass prairies are able to grow and reproduce. (This is from some of my research at Loyola University.)

Find more of the best plants for green roofs on our Pinterest board, and see Richard Hawke’s Plant Evaluation Notes for the plants that performed best on our green roof.

Students in the Chicago Botanic Garden and Northwestern University Program in Plant Biology and Conservation were given a challenge: Write a short, clear explanation of a scientific concept that can be easily understood by non-scientists. This post is part of their series.

60-Second Science: Attack of the Clones!

Those of us who are Star Wars fans know just how powerful genetic cloning can be.

Obi-Wan Kenobi’s discovery of a secret clone army illustrated the power of advanced cloning technology. That army of genetically identical clone warriors went on to become the face of the epic Clone Wars. Meanwhile, in a galaxy much closer, plants are also equipped with the ability to copy themselves as a form of reproduction. This form of asexual reproduction is very common in the natural world, and just as powerful to an ecosystem as a clone army is to a galaxy.

PHOTO: Clone trooper. — This clone may not take over your Garden…

PHOTO: Red Monarda (beebalm). — …but this *Monarda* might!

Clonality is a form of plant growth that results in genetically identical individuals.

Unlike in sexual reproduction, clonal individuals often spread horizontally below ground via unique root systems. Above ground, these plants appear to be distinct individuals, but beneath the soil surface, they remain connected, as clones of the same original plant.

The Pando, or "Trembling Giant," is a colony of clonal quaking aspens roughly 80,000 years old, in Fish Lake, Utah. — The Pando, or “Trembling Giant,” is a colony of clonal quaking aspens (*Populus tremuloides*), roughly 80,000 years old, in Fish Lake, Utah. All the trees are a single living organism sharing one massive root system. Photo by By J Zapell via Wikimedia Commons.

From a plant’s perspective, there are many benefits to clonal growth. For example, in an environment with limited pollinators to facilitate sexual reproduction, it might be better to take matters into your own hands and make a copy of your already awesome self. On the other hand, a vulnerability in one clone (for example, to a fatal fungal outbreak) is just as likely to affect all of the other clones, because they share the same genetic makeup. It is important to note that there are ecological downsides to clonality as well. Many invasive species do well in foreign environments because asexual reproduction enables them to reproduce very quickly. Thus, just as we see in Star Wars, clones can either be a powerful asset or a potent enemy.

Abbey White is a graduate student working with Andrea Kramer, Ph.D., and Jeremie Fant, Ph.D., developing genetically appropriate seed mixes of vulnerable plant species for restoration.

60-Second Science: That’s Not a Seed: Propagating in Saltwater

Most plants hate saltwater. Pour saltwater on your houseplants and, a little while later, you’ll have some wilty plants. But mangroves can grow—and thrive—in saltwater.

You may have seen mangroves if you’ve been to the Florida Everglades or gone to an island in the Caribbean. Mangroves are trees that live in tropical, coastal zones and have special adaptations for life in saltwater. One of these adaptations is in how they reproduce: mangroves don’t make seeds. Instead, they make living, buoyant embryos called propagules (prop-a-gyule).

Mangrove propagules come in different shapes and sizes. Each species has its own unique propagule.

Mangroves produce a huge number of propagules the same way an oak would make hundreds of acorns.

These relatively small propagules could become giant red mangrove trees.

Black mangrove propagules on a branch; their outer coating will dissolve on their journey downstream.

Propagules come in different shapes and sizes. These are from a tea mangrove (Pelliciera rhizophorae) tree. — Propagules come in different shapes and sizes. These are from a tea mangrove (*Pelliciera rhizophorae*) tree.

Normally, trees reproduce with seeds. You’ve probably seen the whirlybirds of maples and acorns of oaks. These seeds can go dormant. They are basically “asleep” or hibernate until something—water, temperature, or physical damage—wakes them up, allowing them to start growing months or years after they are produced.

Here I am with a couple of mangrove specimens. These roots are in water at high tide, but exposed at low tide.

Propagules, on the other hand, don’t have that luxury—they fall off their parent tree, ready to start rooting and growing a new tree. Nature has provided an amazing way for the mangrove seeds to move away from the parent tree: they float.

As the propagules float through the water, they shed their outermost layer and immediately start growing roots. The clock starts ticking as soon the propagules fall—if they don’t find a suitable place to start growing within a certain amount of time, they die. If a mangrove propagule ends its journey at a location that’s suitable for growth, the already-rooting propagule will send up its first set of leaves—cotyledons.

Ocean currents can take propagules thousands of miles away from where they started. A mangrove’s parent tree might be around the corner or around the continent.

Dr. Emily Dangremond is a postdoctoral researcher at the Smithsonian Environmental Research Center and a visiting scientist at the Chicago Botanic Garden. She is currently studying the ecological and evolutionary consequences of mangroves responding to climate change at their northernmost limit in Florida.

60-Second Science: Begone, Buckthorn!

When buckthorn moves in to the ecosystem, it dominates.

Imagine a friend invites you to a dinner party, promising a delicious spread of food and libations. You arrive, excited and hungry, only to find nothing but raw kale, brought by an uninvited guest. Regardless of your feelings about kale, this would be pretty underwhelming. The other guests are obviously disappointed about the monotonous spread. Most people leave, and because most people aren’t eating the kale, the kale continues to dominate the party. Even if someone brought in better foods that more people enjoy, there is no room on the tables. The kale is everywhere!

PHOTO: Buckthorn (Rhamnus cathartica). — Common buckthorn (*Rhamnus cathartica*)

While not a perfect analogy, this anecdote relays the reasons why buckthorn invasion is detrimental to forest ecosystems. The dinner guests are like the other plants and animals that usually live in the woods. They have certain dietary needs, and if those needs cannot be met, they will have to leave and find another place to live. The more one species dominates (kale, or in many local forests, buckthorn) the fewer species can live there, leading to the ecological equivalent of a party that ends at 8:30, just as everyone was arriving. While it may be true that one person at the party really likes kale, it’s hardly fair for the preferences of that person to supersede everyone else’s needs. In the case of buckthorn, many have opposed its removal because that denies robins a berry that they enjoy. However, keeping the buckthorn (which doesn’t belong there in the first place) is like keeping all of the kale on the tables and not allowing for other foods to be served just for that one person. Even more frustrating, the person that likes kale has plenty of other dietary options. Kale isn’t even their favorite food!

To many people, the idea of cutting down trees to help forests grow stronger is counterintuitive. But buckthorn is no ordinary tree. It is an invasive species, meaning that it doesn’t belong in Chicago area forests, and it steals resources from the plants that are supposed to live here. So remember, when you hear people talking about cutting down buckthorn, they are actually doing it to make the habitat healthier and more inclusive in the long term. They are working to replace the kale at the party with better food and drinks, ensuring that all the guests that were invited can have a good time, staying up until sunrise.

Read more about our ongoing buckthorn battle, and see the difference removal makes in restoring an ecosystem.

Bob Sherman is an undergraduate studying environmental science at Northwestern University. His research interests include prairie restoration and how abiotic factors impact prairie and forest ecosystems. He hopes that his research will have a positive impact on ecosystem restoration and management.

60-Second Science: The Janzen-Connell Model or Why Are the Tropics So Diverse?

Peter DeJongh is a first-year master’s student studying land management and conservation in the graduate program at Northwestern University and the Chicago Botanic Garden. His academic focus is on developing strategies to optimize plant and wildlife conservation and restoration. He aims to work in applied conservation or environmental consulting upon completion of his degree.

Imagine a large, beautiful canopy tree standing in the middle of a lush, tropical rainforest. This centuries-old tree produces thousands of seeds every year that densely litter the forest floor around it. Where then would you imagine its seedlings are likely to spring up? Probably in the seed-covered area around the tree right? Well, according to the Janzen-Connell model, you’d be wrong.

Daniel Janzen and Joseph Connell are two ecologists who first described this phenomenon in the early 1970s. They put their exceptional minds to the task and independently discovered that the probability of growing a healthy seedling was actually lower in the areas with the most seed fall. They hypothesized that seed predators and pathogens had discovered the seed feast around the parent tree and moved in, preventing any seeds in the area from growing into seedlings. These predator pests include beetles, bacteria, viruses, and fungi, and have been labelled as host-specific predators and pathogens since they appear specifically around the parent tree, or host.

PHOTO: This Malaysian silverleaf monkey eats fruit as part of its diet, dispersing seeds far beyond the canopy line. — This Malaysian silverleaf monkey eats fruit as part of its diet, dispersing seeds far beyond the canopy line.

Janzen and Connell’s hypothesis shows just how important the animals that eat the seeds are to the parent tree. These primates, birds, and other vertebrates move the seeds to different areas where they can successfully grow without being bothered by those pesky host-specific predators. Without these animal helpers, the forest couldn’t continue to grow, and the world’s most diverse areas would be in serious trouble.

Garden post-grads and scientists are in the field working on restoration efforts in the Colorado plateau, fossil hunting in Mongolia, and filming videos on sphinx moths. Interested in our graduate programs? Join us.